Hi, it's seemingly simple - but causing me a big headache !! I want to buy a camera which will fit into my pocket. I don't think I will ever change a lens so it can have a fixed or changeable lensd. What is the camera that fits the above criterior which takes the best quality photos ?

There is no right answer here. Best check out Gordon's "best camera" site for comprehensive reviews! I'm sure you'll find something that suits your needs!If you want Pocket size go for "point and shoots". Even Mirrorless systems aren't very pockatable when you combine it with a zoom imho.

And last but not least, go to a store and handle the cameras a bit to sit which fits you best!